1. Field of the Invention
This invention relates to novel shell coated zeolite-containing cracking catalysts having enhanced tolerance towards group VIII, and IB metals, particularly towards nickel, and their use in the catalytic cracking of hydrocarbons into products of lower molecular weight. More particularly, this invention is concerned with fluid catalytic cracking (FCC) operations wherein petroleum feedstocks are cracked into products boiling in the motor fuel range.
FCC has been practiced commercially for many years and, due to increasing economic pressures, the feed materials which the FCC unit must process have become heavier and heavier. As is well known in the art, heavier feed materials such as residual oils contain a significantly larger proportion of metals such as nickel and vanadium which have a documented adverse effect on FCC cracking catalysts.
2. Prior Art
There have been many proposals suggested in the patent and technical literature for dealing with heavy metals in FCC, and these may be broadly classified into those which employ process hardware modifications, and those which entail mainly catalyst modifications.
By way of process modifications, feed additives such as antimony have been used to passivate the metals deposited on the catalyst. Although antimony passivation has been fairly successful, it has not been a complete solution to the problems of metal poisoning.
U.S. Pat. No. 4,263,128 discloses an asphalt residual treating process wherein whole crudes or heavy fractions are contacted in a riser with substantially inert microspheres at elevated temperatures. Metals and Conradson carbon are removed from the hydrocarbon feedstock, but catalytic cracking does not take place. While the metals on the inert contacting material reportedly (U.S. Pat. No. 4,781,818) have low activity for coke and hydrogen, the use or inert contacting material to shield cracking catalyst from poisoning by nickel during FCC was not disclosed.
U.S. Pat. No. 4,938,863 discloses a catalytic cracking catalyst which allegedly can tolerate high levels of vanadium and wherein said catalyst can include a silica coated material. The silica coated material is preferably circulated with a separate metal getter according to the disclosure of U.S. Pat. No. 4,938,863.
Another approach to the problem of metals tolerance is to change only the catalyst. The addition of various materials to an FCC catalyst in order to enhance the same for its resistance or tolerance to metals has been proposed numerous times in the patent art. Thus, for example, U.S. Pat. No. 4,485,184 discloses incorporating into an inert solid matrix a sacrificial trap material which allegedly functions to trap metal. This patent does not disclose a shell concept.
U.S. Pat. No. 4,198,320 discloses adding a colloidal dispersion such as silica and/or alumina to a zeolite containing cracking catalyst. This patent does not disclose the concept of a shell.
Shell catalysts per se are not novel and are disclosed in patents such as U.S. Pat. Nos. 4,394,251, 4,793,980, 4,427,577, 4,677,084, 4,378,308, European Patent Application No. 323,735, as well as aforementioned U.S. Pat. No. 4,938,863. However, none of the above patents disclose the particular attrition-resistant coated FCC catalyst of the instant invention.